ShortDocs study guide
Mendelian genetics — core revision notes
Focused visual study guide · shortdocs.ai
How to use this guide
Skim key points first, then read paragraphs for nuance. Drill key terms out loud. On the real product you can open section quizzes and download a PDF with the same structure.
1. What Mendel discovered
Gregor Mendel showed that inheritance follows predictable patterns using pea-plant experiments — discrete factors (genes) are passed from parents to offspring.
Instead of blending traits forever, Mendel found that units of heredity remain distinct across generations. Dominant traits can mask recessive ones in hybrids, yet recessives reappear later in fixed ratios.
His work is the foundation of classical genetics: alleles, dominance, segregation, and independent assortment.
Key points
- Traits are controlled by pairs of alleles.
- Dominant alleles can hide recessive ones in heterozygotes.
- Segregation: allele pairs separate during gamete formation.
- Independent assortment: different genes segregate independently (when unlinked).
Allele
A variant form of a gene (e.g. purple vs white flower).
Genotype
The allele combination an organism carries (e.g. Aa).
Phenotype
The observable trait (e.g. purple flowers).
2. Monohybrid cross & ratios
A monohybrid cross tracks one trait. Crossing two heterozygotes (Aa × Aa) yields the classic 3:1 phenotypic ratio among offspring.
Parent generation pure lines (AA and aa) produce a uniform F1 of heterozygotes (Aa) showing the dominant phenotype. Self-crossing F1 produces F2 with approximately 3 dominant : 1 recessive phenotype, and genotypes 1 AA : 2 Aa : 1 aa.
Punnett squares visualize every gamete combination. Use them under time pressure in exams to avoid ratio mistakes.
Key points
- F1 from true-breeding opposites is uniform heterozygotes.
- Aa × Aa → phenotype ~3:1 (dominant:recessive).
- Test crosses (with recessive) reveal unknown genotypes.
Homozygous
Two identical alleles (AA or aa).
Heterozygous
Two different alleles (Aa).
3. Dihybrid logic (exam checklist)
Two traits at once: if genes assort independently, a dihybrid heterozygote cross (AaBb × AaBb) gives a 9:3:3:1 phenotypic ratio.
Break hard problems into single-trait steps when possible, then recombine. Watch for linkage exceptions — exam questions often flag “genes on the same chromosome.”
Key points
- Independent assortment → 9:3:3:1 for two dominant/recessive traits.
- Always state assumptions (dominance, independent assortment).
- Convert word problems into genotypes before calculating.
Independent assortment
Alleles of different genes segregate independently during meiosis (when not linked).
Inheritance concept map
Check your understanding
Why can a recessive phenotype reappear in the F2 generation?
F1 heterozygotes still carry the recessive allele. When two heterozygotes reproduce, some offspring inherit two recessive alleles and express the recessive phenotype.
What phenotypic ratio is expected from Aa × Aa?
Approximately 3 dominant : 1 recessive, assuming complete dominance and a sufficiently large offspring sample.